Deadly tornadoes have slammed through the US South, killing at least two children, as a massive storm pressed east, threatening to drench communities from New York to Atlanta.
More than 100 million people from the middle of the United States to the east coast were at risk of extreme weather, facing warnings of heavy thunderstorms and another round of tornadoes, said meteorologist Bob Oravec of the National Weather Service's Weather Prediction Centre.
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"So much of the eastern United States faces the risk of severe weather today, and that includes major cities," Oravec said.
Nearly 2000 US flights were cancelled or delayed with most of the trouble at airports in Dallas, Charlotte and Chicago, according to FlightAware.com.
Snow was falling in Chicago on Sunday, with 2.5cm-7.5cm reported in central Illinois, as a result of the storm's cold front.
A total of 17 tornadoes were reported across the south from Texas to Alabama on Saturday and Sunday, Oravec said.
Two children, siblings aged 3 and 8, were killed when a tree fell on the car in which they were sitting in Pollok, Texas, said a spokeswoman for the Angelina County Sheriff's Department.
ABC News reported two more deaths after a tornado ripped through Hamilton, Mississippi.
Tornado warnings remained in effect on Sunday for southeastern Alabama's Barbour and Russell counties, Oravec said.
"It's still a pretty impactful day today. This morning there is a lot of heavy rain moving through Alabama, into Georgia and eastern Tennessee. There are a few tornado warnings and heavy thunderstorm warnings," Oravec said.
Soaking rains could snarl Monday morning's commute on the east coast before the storm moves off to sea.
"The biggest impact rush hour-wise probably will be Boston, around 7 to 8 o'clock in the morning and around New York City around 5 or 6 o'clock, before sunrise," Oravec said.